1. Field of the Invention
This invention relates to the logging of wells during drilling, and more particularly to the wireless telemetry of data relating to downhole conditions.
2. The Prior Art
It has long been the practice to log wells, that is, to sense various downhole conditions within a well and transmit the acquired data to the surface through wireline or cable-type equipment. To conduct such logging operations, drilling is stopped, and the drill string is removed from the well. Since it is costly to stop drilling operations, the advantages of logging while drilling have long been recognized. However, the lack of an acceptable telemetering system has been a major obstacle to successful logging while drilling.
Various telemetering methods have been suggested for logging while drilling. For example, it has been proposed to transmit the acquired data to the surface electrically. Such methods have in the past proved impractical because of the need to provide the drill pipe sections with a special insulated conductor and means to form appropriate connections for the conductor at the drill pipe joints. Other techniques proposed include the transmission of acoustical signals through the drill pipe. Examples of such telemetering systems are shown in U.S. Pat. Nos. 3,015,801 and 3,205,477. In those systems, an acoustic energy signal is sent up the drill pipe and frequency modulated in accordance with a sensed downhole condition. Other telemetering procedures proposed for logging while drilling use the drilling liquid within the well as the transmission medium. U.S. Pat. No. 2,925,251 discloses a system in which the flow of drilling liquid through the drill string is periodically restricted to cause positive pressure pulses to be transmitted up the column of drilling liquid to indicate a downhole condition. U.S. Pat. No. 4,078,620 discloses a system in which drilling liquid is periodically vented from the drill string interior to the annular space between the drill string and the bore hole of the well to send negative pressure pulses to the surface in a coded sequence corresponding to a sensed downhole condition. A similar system is described in the Oil and Gas Journal, June 12, 1978, at page 71.
Wireless systems have also been proposed using lowfrequency electromagnetic radiation through the drill string, borehole casing, and earth's lithosphere to the surface of the earth.
Although the wireless transmission systems just discussed have the potential for increasing the efficiency of drilling operations to offset high operating costs, they are all subject to the disadvantages of transmitting information at a relatively slow rate compared to conventional wireline systems, and are subject to inaccuracies because of the high level of noise usually present in drilling operations.